Operation Of Tube Mill: Complete Technical Guide

The tube mill (the ball mill, the long cylindrical grinding machine that still carries a substantial share of the raw and the finish grinding of the cement industry) is a deceptively simple-looking plant: a rotating drum filled with steel balls that does its work by tumbling impact: the surface simplicity hides the real complexity, and the operation of the tube mill is one of the most behavior-rich disciplines in the process industry: the ball charge, the liner profile, the mill speed, the ventilation, the water injection, the power draw and the product fineness all interact with each other, and the operator who understands the interactions runs the mill at 10-15 percent above the colleagues who do not: the economics of the whole plant stand on the grinding department.

The Complete Cement Technical Package (931 files including the grinding handbooks, the mill dimensioning tools, the Excel calculators, the ball-charge spreadsheets and the operator courses: $249.99 one-time: instant download via the PayPal payment) includes this full operation-of-the-tube-mill file: the guide to the daily and the weekly operation of the ball grinding machine: the article walks the file: the anatomy of the mill, the law of the ball charge, the liners and the compartments, the mill speed and the power, the ventilation and the temperature, the water injection, the output control and the fineness, the maintenance of the charge and the troubleshooting of the pictures that every miller recognizes: the reader finishes the guide with the same operating reasoning as the experienced plant engineers.

The tube mill carries the tradition and the bulk of the finish grinding of the world’s cement, even in the time of the vertical roller mills, because of its flexibility toward the feed moisture, the additives, the blended cements and the grinding work: this article presents the operation of the tube mill through the numbers of the industry: the working speed (70 to 75 percent of the critical speed), the ball charge of the compartments (60-120 mm in the first, and 15-50 mm in the second), the ventilation of 0.5 to 1.5 m/s through the mill, the water injection of up to 1.5 percent of the feed, and the specific energy of the finish grinding (30-40 kWh/t): these numbers give the reader the precise space the operation moves in.

1. The Anatomy of the Tube Mill: The Compartments, the Shell and the Drive

The tube mill is a horizontal cylinder of the diameter typically 3.0 to 4.8 m and the length 9 to 17 m, divided by internal diaphragms into two or three compartments:

• The first compartment (the coarse grinding): 25-30 percent of the length, the balls of 60-120 mm, the wave or the step liners, the task: the impact crushing of the feed particles of 10-30 mm down to the state of 1-2 mm;
• The diaphragm: the partition walls between the compartments with the slots through which the material passes; the smallest slot sets the transfer size between the compartments;
• The second compartment (the fine grinding): the remaining 70-75 percent of the length, the balls of 15-50 mm, the classifying liners which automatically sort the media by size along the length, the task: the surface grinding to the final fineness;
• The drive: the gear ring and the pinion, or the central drive, the motor of 1,500 to 5,000 kW, the starting currents limited by the couplings;
• The trunnion bearings: the hollow journals seated on the hydrodynamic bearings, the white metal, the lubrication of the film at 40-60 C;

The two-compartment mill dominates the cement finish grinding because the separation of the functions (the impact then the attrition) matches the physics of the size reduction: the file details the cutting of the compartments with the rule of the thumb for the age of the plant: the first compartment of 4.5 to 5.5 m in a 4.2 x 14 m mill and the second of 7.5 to 8.5 m.

2. The Ball Charge: The Foundation of the Grinding

No parameter of the mill influences the output as directly as the charge of the media: the charge is defined by three numbers:

• The filling degree (the ratio of the volume of the balls to the compartment volume): the operating mills run at 28-34 percent of the compartment filling, the optimum of the output; the figure is measured by the stop of the mill or by the sound of the first falls;
• The ball size distribution: the graduated load: the large balls for the impact and the small for the surface area: the first compartment in the range of the 60-100 mm, the second below 40 mm;
• The ball quality: the forged balls of 0.6-0.7 percent carbon steel, or the high-chrome cast varieties: the wear rate of the chrome media 20-40 grams per ton of cement versus 150-300 g/t for the forged steel;

The ball charge is measured by the mill stop-rule (the visible ball level under the discharge) and by the charge balance over time: the plant monitors the charge monthly and the top-ups at 2-5 day intervals: the file provides the registration tables of the charge of every mill of the system.

3. The Speed of the Mill and the Motion of the Media: The Physics of the Fall

The single ball inside the rotating mill follows a path determined by the speed of the shell: at the normal operating speed the media rise with the shell, orbit, and recap tire in a flow that resembles the surf of the sea:

• The cascade motion: in the rolling phase, the balls tumble over each other, grinding by the attrition and the shear at the contact surfaces: the mode of the fine compartments;
• The cataract motion: at the higher speeds, the media are lifted higher and fall freely through the cylinder: the cataract impact breaks the largest particles: the mode of the coarse compartment;
• The critical speed: the rotational speed at which the media centrifuge against the shell: Ncr = 42.3 / sqrt(D) rpm with D in meters: for the 4.2 m mill, Ncr = 20.6 rpm;
• The operating speed: the working speed of 70-75 percent of the critical, i.e. 14.5 to 15.5 rpm at the 4.2 m: the efficiency of the mill is the maximum in this zone;

The speed table of the typical mills of the file: 3.2 m: 16.6 rpm; the 3.6 m: 15.7 rpm; the 4.0 m: 14.9 rpm; the 4.2 m: 14.5 rpm: the engineer uses the curve of the power versus the speed to set the operating point of the mill, between the rated output and the acceptable vibration.

4. The Liners: The Wear Parts That Shape the Motion

The liner system of the mill has the double mission: the protection of the shell and the steering of the material:

• The lifting liners: the cascade (wave) liners of the first compartment with the high lift of 60-80 mm: the shelf on which the charge falls: the liners of the Ni-Hard or the high chrome with a life of 12-18 months typical;
• The classifying liners: the wedged profiles of the second compartment which roll the media toward the discharge ends, the large balls near the inlet and the small ones near the outlet: the fine grinding performance 10-20 percent higher than the flat profile;
• The head liners: the inlet and the outlet heads protected with the matching steel: the life of the heads 10-15 years and of the shells up to the same, whereas the charge liners are replaced at every 3-5 years;
• The fixing systems: the bolted shell liners, the side entry replaceability; the replacement strategy of the package (the full set vs the sectional) decided by the condition of the monitoring;

Note the interplay: the worn flat lining lowers the lifting and the mill power by 5-10 percent and slips the charge: the annual inspection of the lift heights and the planning of the replacement at 60-70 percent of the wear: the file discusses the energy cost of the worn liners with the worked example.

5. The Ventilation of the Mill: The Air That Carries the Heat and the Product

The tube mill must be ventilated: the air drawn through the shell conveys the fine particles, removes the steam of the moisture and keeps the internal temperature in the safe band:

• The flows: the ventilation of the finish mill 0.5 to 1.5 m/s of the air velocity and the total of 0.7 to 1.5 m3 of air per ton of the feed to the mill;
• The mill temperature: the outlet gas 85-110 C in the finish grinding at the peak of the summer, 90-120 in the raw mills with the drying;
• The water injection: the interior spraying of the water (up to 0.5-1.5 percent of the feed) in the summer or at the high fineness to hold the temperature under 110 C and to protect the diaphragm;
• The collection: the exit gas goes to the cyclone/filter and the product is collected: the system must be balanced so that the mill stays under the slight negative pressure at the inlet (the -50 to -150 Pa);

The classic symptom of the ventilation loss is the “choked” mill: the exit temperature rises, the output falls, the fineness degrades and the current climbs: the file guides the check of the air distribution, the flows, the filter conditions and the damper positions: the operation of the overseas without a ventilation audit is the operation in the dark.

6. The Grinding Aids and the Water Injection: The Chemical Modifiers of the Mill

The grinding aid is the modern technique of the cement mill: the organic surfactant (typically the amine or the glycol formulations) sprayed at the mill inlet at the doses of 100-500 g per ton of material:

• The mechanism: the aid adsorbs on the newly created surfaces and reduces the re-agglomeration of the dry fines (the coating and the wall packing, the blinding of the screens): it also improves the flowability of the mill intermediate and the separator feed;
• The results: the typical 5-10 percent of the output gain at the same fineness, and the corresponding energy saving: the Blaine of the product rises at same energy or the same at the lower;
• The quality dossier: every additive of the plant is checked by the quality department: the effect on the strength and the water demand of the concrete is tested for the 28 days before the adoption;
• The water: the internal water is not a grinding aid but the temperature controller: the number: the cooling of the mill demand the 1 kg of water per 1.5-2 kg of the mill temperature drop: practical ceiling 1-1.5 percent of the feed:

The file separates the two clearly and gives the trial protocol of the aids: the 30-day production comparisons with the fixed feed, the fineness and the output recorded daily: the standard statistics of the small sample, to protect the plant from the false claims of the suppliers.

7. The Power Draw and the Energy Management of the Mill

The electrical power of the mill is the largest single load of the automated grinding system, and its reading is the dashboard of the mill:

• The expected draw: the design of the 4.2 x 14 m mill is about 3,400-3,500 kW at the rated performance and the needed charge: the daily vs the design deviation of 5 percent signals the charge loss or the liner;
• The band of the operation: the motor at the 85-95 percent of the rating in the stability: the excursions above signal the overfeed, the excursions below the empty or the choked mill;
• The specific energy: the kWh/t of the product is the course of the mill: 25-40 kWh/t finished cement at the 3,300-3,700 cm2/g; the file tabulates the typical targets by the cement type;
• The economics: 1 kWh/t saved on the finish grinding of the 1.0 Mt/y plant at 0.07 USD/kWh saves 70,000 USD/y: the customer of the file energetically the power;

The trend of the specific energy vs the fineness is the visible illness panel: the file includes the spreadsheets of the monthly KPIs with the graphs, so the drift of the mill (the worn liners, the lost charge, the varying feed) is caught in the week, not the year.

8. The Control of the Output and the Fineness: The Mill, the Circuit and the Feed

The mill rarely works alone: it sits in a closed circuit with the separator (the file on the operation of separators treats that part), and the joint control is:

• The feed control: the fresh feed rate regulates the mill power: the overfeeding reduces the power of the mill (the charge rises, the falls of the media are dampened): the sign of the overloaded mill is the drop of the current;
• The fineness control: the separator speed sets the target Blaine: the daily adjustment of the rotor speed of the separator and the hold of the mill temperature by the ventilation and the water;
• The campaigns: the production of the different cements (the CEM I, the CEM II with the limestone, the PPC) is organized in the campaigns to minimize the changes of the settings and the transition losses: the file gives the campaign rules of the grinding plant;
• The instrumentation: the online particle size analysers in the modern plants measure the fineness continuously and the adjustments follow: the sampling rate 5-15 minutes:

Adjusting the output of the mill is not chasing the numbers but holding the balance of the loop: the file explains the dynamic behavior of the circuit (the response of the mill to the feed changes is the 10-20 minutes: to the separator changes the 30-60 minutes) so that the operator learns to wait the correct time before the next move: the classic error of the new operator is the oscillation caused by the constant too-fast correction.

9. The Thermal Control and the Water of the Mill: The Details of the Practice

The temperature field inside the mill is the hidden dimension of the operation:

• The temperature limits: the outlet gas of the finish mill should stay under 110-115 C, the cement under 100-105 C at the silo: the higher values degrade the quality of the gypsum (the loss of the bound water above 105 C) and the flow of the cement;
• The water injection layout: the water is injected at the discharge zone or through the hollow shaft: the drop of the temperature by the evaporation, the injection of the nozzles frequently at the hours of the high load;
• The measurements: the temperature probes in the inlet and the outlet housings, the infrared on the shell, the temperatures of the bearings: the differentials in the readings indicate the mill internals;
• The winter vs the summer: the same mill behaves differently: the seasonal adjustments of the ventilation and the water are in the file: the cold air fills the moistur

The thermal management is decisive for the two chronic illnesses: the gypsum dehydration (the loss of the sulphate function, the false set of the concrete) and the coating of the grinding media (the dust layer that destroys the char efficiency): the file treats both with the full procedure of the prevention.

10. The Maintenance Program of the Tube Mill: The Calendar and the Checks

The tube mill is a rotating heavy machine and its maintenance is the discipline of the register:

The annual stop is the “workshop of the mill”: the complete measure of the charge, the replacement of the consumed balls, the re-coating inspection, the repair of the slots: the file contains the two-week plan of the annual stop with the work list and the manpower planning.

11. The Troubleshooting of the Tube Mill: The Symptoms, the Causes, the Cures

The daily life of the mill produces the well-known symptoms, and the file tables them all:

Then the file explains the “king” of the mill problems, the plugging of the diaphragm (the putty of the fine material and the moisture sealing the slots): the detection: the fall of the output with the stable power, the rise of the pressure: the cure: the careful partial opening of the mill, the air-swept, the drying of the feed: the procedures of the extreme case are included for the safety of the crew.

12. The Safety of the Tube Mill: The working in the confined area and the stored energy

The tube mill is also one of the most dangerous machines of the plant when the safety rules are not observed: the kinetic energy of the charge, the hot components, the dust and the confined space:

• The lock-out/tag-out: the drive has to be locked out in the stop: the rotational potential of the charge makes the entry strictly forbidden before the verification of the blocked rotation (the mechanical locking of the gear ring);
• The confined space: the interior of the mill is a confined space with the poor ventilation: the gas tests (O2, CO, the explosive gas), the harness retrieval, the attendance: the standard procedures are in the file;
• The thermal: the temps inside reach the 100 C and above: the cooling period before the entry: the temperature measurements of the interior:
• The media handling: the grinding balls of several tons: the mechanical removal systems, the slings certified, the visual of the stored energy of the charge as the pendulum risk;

The accident statistics of the industry highlight the mill maintenance: the file therefore treats the safety as the part of the normal operation: the permission to work, the isolation of the energy, the verification of the zero energy, and the recovery of the entry team: the culture of the safe maintenance is the first discipline of the professional miller.

12. The Grinding Circuits of the Different Cements: The Flexibility of the Tube Mill

The tube mill serves every cement type of the plant, and the operational hand of the miller adapts the machine to the material: the table of the file collects the working points of the different products:

The flexibility to switch between the products with the same mill is the strategic asset of the cement plant, and the file explains the switch costs: the change of the feed blends, the flushing of the mill (the 30-40 minutes with the new feed before the product is accepted), the separator re-calibration and the quality samples: the manager who plans the campaigns well keeps the mill at the maximum average output even with the many products of the day.

13. The Frequently Asked Questions

Why does my mill operate at a lower output than the design?

The cause is found in the five checks: the ball charge (the filling and the gradation), the liners (the lift), the ventilation (the flow and the gap), the feed (the size and the moisture), the separator (the cut and the bypass): the diagnosis path of the file takes the reader through these five in the hour, and the measured data decides: the design output returns with the charge of the lining and the feed discipline.

At what filling degree should I run the mill?

The typical filling between 28-34 percent of the volume at the first compartment and 30-33 in the second: the actual optimum depends on the speed, the liners and the product: the file has the table of the empirical suggestions of the mills 3.0-5.0 m and the method of the measurement (the stopping, the measuring, the marking the final level):

The vertical mill vs the tube mill for my new project?

The mill selection table of the package: the vertical mill: the lower power of the raw grinding (16-24 kWh/t vs 24-30 of the ball), the higher sensitivity to the moisture, the simpler circuit: the ball mill: the higher flexibility of the additives and the finished quality of the slag cements: the finish grinding of the modern best practice still uses the ball of the compartment or the combi (pre-grinder + ball); the file details both with the cost of the total project;

How do the grinding aids fit in the operation?

The aids carry the energy of the mill: the studies of the file show the 5-10 percent of the energy and the output gain, the full conditioning (the better retention), the flow improvement: the dosage is optimized per the product because the over-dosing (the above the 500 g/t) wastes and can degrade the concrete: the correct dose is the one that shows the gain in the 30-day trial, not the one the brochure sold.

What is the meaning of the coating and how do I remove it?

The coating is the layer of the fine material sticking to the media and the liners: it grows with the fine grinding, the moisture above the 1-2%, and the weak vent: the cures: the vent increase, the water wash (in the water-cooled mills), the grinding aid at the higher dose for the days, and the periodic “empty driving” of the mill: the file describes the operations in the order of the risk to the equipment.

15. The Conclusion: The Mill That Pays Its Operator

The operation of the tube mill is presented in the package as it is in the real plants: the machine of the physics, the pressure of the economy, and the skill of the human being: the operator who understands the ball paths, the charge levels, the ventilation and the water can take the emphasis plant from 95 to 108 t/h at the same motor and the same fineness: the mill of the case study of the file tells exactly this story in the numbers of its P&L pages.

The file of the operation of the tube mill is one of the 931 files of the Complete Cement Technical Package: the complete collection covers the mill design, the SEPARATOR operation, the mills of the raw and the finished, the working examples, the Excel calculators and the maintenance planning: the full package: the one-time 249.99: the instant download: the page of the PayPal above: the cement professional: the mill: the future of the profession: the money well of the technical career.

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